The present invention relates to a rapid heating type glow pulg for starting a Diesel engine and, more particularly, to a self-control rapid heating type glow plug.
It has been known generally that a Diesel engine is difficult to start at low temperatures. In order to overcome this problem, it has been usual to provide a glow plug in the engine's cylinders or an auxiliary combustion chamber thereof for increasing the temperature in the cylinder or the auxiliary combustion chamber prior to starting. For this purpose, the glow plug must have a rapid heating characteristic. Recently, there has been a tendency for glow plugs to be used not only for starting but also during normal engine operation to stablize the fuel combustion in the cylinder. If a glow plug is used continuously in such a manner, the glow plug must have an improved durability.
For this purpose, a rapid heating type glow plug has been developed which is composed of a sintered ceramic body and a tungsten heating wire. Tungsten is heat durable, and thus there is little chance of thermal breakdown thereof at high temperatures. However, there is a possibility of the ceramic body being cracked by thermal shock due to rapid heating. In order to obviate the cracking problem of the ceramic body, it has been the practice to provide some auxiliary means such as a controller for controlling the current flow through the heating wire.
As another approach, a so-called self-control type glow plug has been proposed, which is composed of a ceramic body having a tungsten heating wire embedded therein and a resistor connected in series with the heating wire. The resistor, which is implemented as a wire made of a material such as nickel, tungsten or molybdenum, has a positive resistance temperature coefficient larger than that of the heating wire so that, during rapid heating, the resistance of the resistor increases rapidly to reduce the heating current and thereby prevent overheating of the heating wire. In such a self-control type glow plug, it is desired, for realization of a satisfactory self-control function, that there be a large difference in the temperature resistance coefficient of the resistor between room temperature and, for instance, 1000.degree. C. For example, if the resistor is made of nickel for which the temperature resistance coefficient at 1000.degree. C. is about 6 to 7 times that at room temperature, the heating wire connected in series with the resistor should have temperature resistance coefficient ratio of 4 or smaller. The term "temperature resistance coefficient ratio" herein means the ratio of the temperature resistance coefficient (change in resistance per degree change in temperature) at 1000.degree. C. to the temperature resistance coefficient at room temperature.
A heating wire made of a material such as Fe--Cr alloy or Ni--Cr alloy can satisfy the above requirement. Metal glow plugs having such a heating wire disposed in an insulating powder filling a metal sheath have been built and tested. In such a metal glow plug, however, since the melting point of the heating wire is relatively low, it cannot withstand the sintering temperature of the ceramic material. Further, even if the heating wire could withstand such a high temperature, it cannot be successfully used with the ceramic body due to the large difference in thermal expansion coefficients therebetween.
For these reasons, tungsten heating wire has been used in conjunction with a ceramic body. The tungsten of the heating wire is of 99.9% purity or more, and accordingly the temperature resistance coefficient thereof is large. Hence, it is impossible to provide a large difference in temperature resistance coefficients between the resistor and the heating wire so that the self-control function of such a glow plug has not been sufficient.